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Journal of Molecular Structure 831 (2007) 187–194

Crystal structure and spectroscopic analysis of the asymmetricsquaraine [(2-dimethylamino-4-anilino)squaraine]

Carlos Eduardo Silva a, Renata Diniz a, Bernardo L. Rodrigues b,Luiz Fernando C. de Oliveira a,*

a Nucleo de Espectroscopia e Estrutura Molecular, Departamento de Quımica, Universidade Federal de Juiz de Fora, Campus Universitario,

Juiz de Fora, MG, 36036-900, Brazilb Instituto de Fısica de Sao Carlos, Universidade de Sao Paulo, Sao Carlos, SP, Brazil

Received 20 June 2006; received in revised form 1 August 2006; accepted 1 August 2006Available online 15 September 2006

Abstract

The asymmetric squaraine (2-dimethylamino-4-anilino)squaraine (ADTCH3) has been synthesized and investigated by single crystalX-ray diffraction and vibrational spectroscopy. This compound crystallizes in a monoclinic space group P21/n and has four molecules perunit cell. The nitrogen atom of the aniline ring and the oxygen atoms of the four-membered ring are involved in a medium-strengthhydrogen bonded interaction, giving rise to a dimer design where the N� � �O distance is 2.880(2) A. Two types of molecular arrangementscan be observed in the crystal packing, forming a wave design parallel to the c crystallographic axis. The cyclobutene rings are effectivelyinvolved in a p-stacking interaction where the interplanar and centroid–centroid distances are 3.24 and 3.58 A, respectively. Vibrationalspectroscopy (Raman and infrared) of ADTCH3 shows that the CO stretching modes from the oxocarbon and the aniline componentsare little affected by molecular substitution. The Raman spectrum of ADTCH3 shows intense bands at 1604 cm�1 and 1595 cm�1, relatedto the m(CC) + m(CO) and m(CC) + m(CN) vibrations, which can be used to identify the electronic delocalization in the cyclobutene ring.The infrared spectrum shows a medium-weak band at 3120 cm�1 and a broad band at 3197 cm�1 which can be tentatively assigned to in-phase and out-of-phase stretching of the NH bond of the dimer species. These results are in good agreement with X-ray crystal datawhich show the presence of this arrangement in the solid state.� 2006 Elsevier B.V. All rights reserved.

Keywords: Asymmetric squaraine; Oxocarbon derivatives; Hydrogen bond; Crystal structure; Vibrational spectroscopy

1. Introduction

Oxocarbon ions, of the general formula [(CnOn)2�], havea high molecular symmetry (Dnh) and effective electronicdelocalization [1,2]. These characteristics are very interest-ing and important for several studies mainly related tostructural and spectroscopic investigations. Replacementof the oxygen atoms by other atoms or groups in oxocar-bon ions gives rise to the well-known pseudo-oxocarbons[3]. Mesoionic derivatives of squaric acid [H2C4O4], whichhave a nitrogen atom in their structure, are termed squa-

0022-2860/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.molstruc.2006.08.001

* Corresponding author. Tel.: +55 32 3229 3310; fax: +55 32 3229 3314.E-mail address: luiz.oliveira@ufjf.edu.br (L.F.C. de Oliveira).

raines. The general representation of this species is shownin Fig. 1. These species have sharp and intense absorptionbands in the long wavelength range of the visible and near-infrared regions [4] and have been investigated in recentyears due to their interesting chemical and physicalproperties, as for instance, substrates for photovoltaicinvestigations [5], photoconductors and photoreceptors[6], and non-linear optical applications [7]. Solid-stateinvestigations of these species are very important for theunderstanding of their photophysical properties. In spiteof all the spectroscopic analysis of such compounds, fewvibrational investigations have been undertaken and mostof these have been in the infrared. It is well known fromthe literature that squaraines possess intense fluorescence

Fig. 1. General molecular structure of squaraine molecules.

188 C.E. Silva et al. / Journal of Molecular Structure 831 (2007) 187–194

emission which renders the vibrational Raman study ofthese systems very difficult using visible excitation. Canoand coworkers [8] have investigated the Raman spectraof squaraines inside zeolites and have concluded that thecomparison of the vibrational spectra gives a good indica-tion of the purity of the organic guest molecules; however,in that paper the authors do not present a discussion abouttheir band assignments. In a very recent paper, Lopes et al.[9] describe the resonance Raman profile of two squarainesin organic solutions, and have shown that the chemicalequilibrium in very dilute solutions is more important thanthe aggregate phenomenon for the analysis of the chromo-phoric moiety, as proposed hitherto in the literature. Infra-red spectra of cis- and trans-dimethylsquaraines have beendescribed by Lunelli and coworkers [10], who investigatedthe water exchange between them and the partial pressureof water over the hydrated compounds; however, onceagain, no specific vibrational assignments were reportedby the authors.

In non-linear optical research the recognition of organicmolecules that have high dipole moments, asymmetric con-jugated p-electron systems and non-centrosymmetric crys-talline structures is important. In this sense, squarainespecies are good candidates for this kind of investigation,and the knowledge of their solid state properties as provid-ed by crystallographic and spectroscopic analyses is vital.Unsymmetrical squaraines have been investigated by Law[11], who concluded that this kind of squaraine exhibitedan efficient radiationless decay property compared withsymmetrical and pseudo-unsymmetrical squaraines. Somecrystal structures of squaraines [4,5,10,12,13] and their lan-thanide complexes [14] are described in the literature;among these, only a few structures [10,12] contain the sub-stitution of an oxygen atom in the squarate ring by nitro-gen atoms and these therefore provide the system ofinterest for the current investigation. Lunelli and cowork-ers [10,12] have investigated the crystal structure and vibra-tional spectra of 3,4-bis(dimethylamino)-3-cyclobutene-1,2-dione (DMACB), named the cis structure, and the trans

analogue [bis(dimethylamino)squaraine] in three differentconfigurations, namely the anhydrous isomer (SQ), andtwo hydrated isomers (SQ2 and SQ4). The crystal packingof DMACB shows columns of stacked molecules parallelto the crystallographic axis a, in SQ2 these are parallel tothe b-axis and in SQ and SQ4 parallel to the c-axis. In allthese compounds the p-stacking interaction presenting anaverage of the topological parameters [15] was observed,giving values of 3.60(9) and 3.34(1) A for the centroid–cen-

troid distance and interplanar distance, respectively. InSQ2 was observed the biggest shift of centroids (1.51 A)was observed, and the smallest observed in DMACB(0.44 A). The smallest interplanar distance was observedin SQ, which suggests that trans-substitution could befavorable for this kind of intermolecular interaction.

In this work an unsymmetrical squaraine, (2-dimethyla-mino-4-anilino)squaraine (ADTCH3), is synthesized andcharacterised by single crystal X-ray diffraction, UV–visi-ble and vibrational (Raman and infrared) spectroscopy.The main purpose of this investigation is to identify thesymmetry of the unit cell, the intermolecular interactionspresent in the solid state, and their influence upon the crys-tal packing from the vibrational spectroscopic point ofview.

2. Experimental

2.1. Synthesis

ADTCH3 was synthesized as described by Neuse andGreen [16], based on the reaction of squaric acid (Aldrich)and aniline (Aldrich) in dimethylformamide at 145 �C for90 min in the molar ratio of 1:2. The yellow solid obtainedwas added to 15 mL of hot methanol (60 �C) followed byfiltration. Single crystals of ADTCH3 were obtained byslow evaporation of the methanol solution at room temper-ature. Elemental analysis of C12H12N2O2 (216.26 g mol�1)gave, calcd: C, 66.6; H, 5.60; and N, 12.9%; found: C,62.8; H, 5.51; and N, 11.3%.

2.2. X-ray diffraction

Single crystal X-ray data were collected in a NoniusKappa CCD diffractometer with MoKa (k = 0.71073 A)at room temperature. Data collection and reduction andcell refinement were performed by DENZO and SCALE-PACK programs [17]. The structure was solved and refinedusing SHELXL-97 [18]. An empirical isotropic extinctionparameter x was refined to 0.014(6), according to themethod described by Larson [19]. The structure was drawnby ORTEP-3 for Windows [20]. Hydrogen atoms werelocated from Fourier difference maps. Anisotropic dis-placement parameters were assigned to all non-hydrogenatoms. CCDC 292491 contains the supplementary crystal-lographic data for this compound. All these data can beobtained free of charge at www.ccdc.cam.ac.uk/conts/re-trieving.html or from the Cambridge CrystallographicData Centre, 12, Union Road, Cambridge CB2 1EZ, UK[Fax: (internat.) +44 1223/336 033; e-mail: deposit@ccdc.cam.ac.uk].

2.3. Raman spectra

Fourier-transform Raman spectroscopy was carried outusing a Bruker RFS 100 instrument and a Nd3+/YAG laseroperating at 1064 nm in the near infrared with a CCD

Table 2Selected bond distances and bond angles, and geometric parameters of thehydrogen bond in ADTCH3

Bond distance/A

C1–C2 1.432(2) C1–N1 1.339(2)C2–C3 1.452(2) C2–O1 1.240(2)C3–C4 1.459(2) C3–N2 1.315(2)C4–C1 1.464(2) C4–O2 1.232(2)

C5–C6 1.384(2) N1–C5 1.411(2)C6–C7 1.381(3) N1–H1 0.950(17)C7–C8 1.382(3) N2–C11 1.457(2)C8–C9 1.372(3) N2–C12 1.464(2)C9–C10 1.373(3)C10–C5 1.389(3)

Bond angle/�C2–C1–C4 91.52(13) C2–C3–C4 90.92(13)C2–C1–N1 127.65(15) C2–C3–N2 134.53(15)C4–C1–N1 140.81(15) C4–C3–N2 134.54(16)C1–C2–C3 89.53(13) C3–C4–C1 88.04(13)C1–C2–O1 135.00(16) C3–C4–O2 134.61(16)C3–C2–O1 135.47(17) C1–C4–O2 137.34(17)

C1–N1–C5 129.30(14) C3–N2–C11 121.34(13)C6–C5–N1 122.48(15) C3–N2–C12 121.89(14)C10–C5–N1 118.05(15) C11–N2–C12 116.71(14)

C6–C5–C10 119.45(15) C7–C8–C9 119.09(17)C5–C6–C7 119.06(16) C8–C9–C10 120.40(18)C6–C7–C8 121.41(17) C9–C10–C5 120.57(17)

Torsion angle/�C1–N1–C5–C6 �3.9(3) O1–C2–C3–N2 2.5(4)C1–N1–C5–C10 177.62(18) O1–C2–C1–N1 �2.9(3)C11–N2–C3–C2 6.0(3) O2–C4–C1–N1 1.2(4)C12–N2–C3–C4 4.7(3) O2–C4–C3–N2 �0.4(3)

Hydrogen bond N–H/H–O/A N� � �O/A N–H–O/�

N1–H1–O1 0.95(2)/1.96(2) 2.880(2) 162.1(2)

C.E. Silva et al. / Journal of Molecular Structure 831 (2007) 187–194 189

detector cooled with liquid N2. Good signal-to-noise ratioswere obtained from 2000 scans accumulated over a periodof about 30 min and 30 mW of laser power at a spectralresolution of 4 cm�1. All spectra were obtained at leasttwice to show reproducibility and no changes in band posi-tions and intensities were observed.

2.4. Infrared spectra

Infrared spectra were obtained using a Bomem MB-102spectrometer fitted with a CsI beam splitter, in the form ofKBr disks and with a spectral resolution of 4 cm�1. Goodsignal-to-noise ratios were obtained from the accumulationof 128 scans.

2.5. Electronic spectrum

UV–vis spectra of dimethylformamide solutions ofADTCH3 were obtained with a Shimadzu UV-1601PCspectrometer using a 2.0 nm spectral resolution and a scanrate of 500 nm min�1.

3. Results and discussion

Crystal data of ADTCH3 are listed in Table 1 and somegeometrical parameters are shown in Table 2. Fig. 2 dis-plays the crystal structure of ADTCH3 and the crystalpacking is shown in Fig. 3.

Despite the molecular asymmetry, ADTCH3 crystallizesin a P21/n space group, which produces a centrosymmetricunit cell containing four molecules. The anilino system(C5–C10, N1) and the four-membered oxocarbon ring(C1–C4, O1, O2, N1, N2) are essentially coplanar, withmean deviations of planarity for these two rings of

Table 1Crystal data of ADTCH3

Formula C12H12N2O2

Formula weight/g mol�1 216.23Crystal system MonoclinicSpace group P21/n

a/A 5.7650(2)b/A 8.0422(6)c/A 22.7836(14)b/� 94.169(4)V/A3 1053.53(11)Z 4

Crystal size/mm 0.12 · 0.12 · 0.36dcalcd/g cm�3 1.357l(MoKa)/cm�1 0.095

No. de reflections 7826R(int)/R(r) 0.0660/0.0679

No. of unique reflections 2398No. of observed reflections, F 2

o > 2sðF 2oÞ 1689

No. of parameters refined 152R 0.0685wR 0.1150S 1.090

RMS peak (e� A�3) 0.039

Fig. 2. ORTEP view of the ADTCH3 crystal structure. Ellipsoids aredrawn at the 50% probability level, except for hydrogen atoms which arerepresented by circles of arbitrary radius.

0.017 A. The molecule overall is nearly planar with aninterplanar angle between the rings of 8.96�. The CC bonddistances in the four-membered ring are almost similar;the average CC bond distance is 1.452 A and the largestdifference in CC distance is 0.032 A, showing a degree ofelectronic delocalization in the molecule. A similar obser-vation has been made for other oxocarbon structures, suchas salts of ammonium squarate [21]. For the ADTCH3

Fig. 3. Crystal packing of ADTCH: (a) showing hydrogen bond design,and (b) the wave arrangement along the a crystallographic axis.

190 C.E. Silva et al. / Journal of Molecular Structure 831 (2007) 187–194

molecular formula, it is possible write the four canonicalstructures which can be seen in Fig. 4. Comparisonbetween the carbon–carbon and carbon–nitrogen bond dis-tances presented in Table 2 shows that C1–C2 is the small-est CC bond distance and that C1–N1 is bigger than theC3–N2 bond distance. As C1–C2 refers to the oxocarbonring and C3–N2 to the aniline moiety, it seems that thecanonical structure a depicted in Fig. 4 contributes a littlemore than the others towards the resonance structure. Inthe benzene ring the average and the biggest difference inbond distance being 1.380(3) A and 0.017 A, respectively.The CO and CN bond distances of the four-membered ringexhibit a double bond character, and the average distancesof these bonds are 1.236(2) and 1.327(2) A, respectively.

Fig. 4. Canonical struc

The C–N bond distances of the amino groups are similarin value, with an average bond distance of 1.444(2) A.The nitrogen atom of the aniline ring is protonated, form-ing a zwitterion species (Fig. 2). As can be seen in Fig. 3a,the oxygen atom of the four-membered ring is involved in amedium-strength hydrogen bond to the NH group ofanother molecule, giving rise to a dimer. The N� � �O dis-tance here is 2.880(2) A and the four-membered rings ofthe two molecules are coplanar with only 0.0282 A of devi-ation in planarity. In the crystal packing, two types ofmolecular arrangements forming a wave design parallelto c crystallographic axis (Fig. 3b) are observed and theangle between the molecular planes is 67�. A similar molec-ular arrangement has been observed in anhydrous bis(dim-ethylamino)squaraine and its cis-isomeric species [12], withsmaller angular planes. In this cis-isomer, the anglebetween the molecular planes is about 17� and in trans-an-alogue it is 36�.

In Table 3 are listed the geometric parameters of thep-stacking interactions [15] for ADTCH3. The centroid–centroid and interplanar distances are less than 4.0 A, indi-cating that this compound presents an effective p-stackinginteraction between the four-membered rings. As can beseen from Table 3, ADTCH3 shows the smallest interpla-nar distance among the squaraine molecules, similar to thatobserved in the potassium squarate salt [22]. However, thedistance between the ADTCH3 ring centroids is similar tothat observed for the dimethylamine isomers [10,12]. Thisresult suggests that the four ring arrangement is similarwhen compared to other squaraine molecules, but inADTCH3 the molecules are closer and are now similar tothe structures of the squarate salts of the alkali metal ions[12,23]. The horizontal translation of the rings (i.e. the shiftbetween the ring centroids of two molecules from different

tures of ADTCH3.

Table 3Geometrical parameters of the p-stacking interaction of some squaraine-like molecules and squarate salts

Compound Dist. 1/A a Dist. 2/Aa Shift/Aa Reference

Na2C4O4 – p1 3.30 3.55 1.28 [23]Na2C4O4 – p2 3.33 3.61 1.31 [24]

3.14 3.38 1.17K2C4O4 3.27 3.30 0.34 [22](NH4)2C4O4 3.41 3.71 1.26 [21]ADTCH3 3.24 3.58 1.27 This workSQ 3.32 3.49 0.95 [12]SQ2 3.35 3.74 1.51 [10]SQ4 3.35 3.55 0.83 [10]DMASQ 3.33 3.60 1.24 [12]

3.35 3.63 0.44

a Dist. 1 – interplanar distance, Dist. 2 – distance between centroids,Shift – horizontal translation, SQ – bis(dimethylamino)squaraine anhy-drous, SQ2 – SQ dehydrated, SQ4 – SQ tetrahydrated, DMASQ – 3,4-bis(dimethylamino)-3-cyclobutene-1,2-dione.

C.E. Silva et al. / Journal of Molecular Structure 831 (2007) 187–194 191

stacks) shows that the rings are shifted by 1.27 A, a valuethat is close to that of the sodium squarate salt [23]. Theanalysis of geometrical parameters of Table 3 shows thatthe ADTCH3 possesses the most effective p-stacking inter-action among squaraine-like molecules. For the alkali met-al squarate salts, the most effective interaction wasobserved in the potassium salt and the weakest in theammonium salt, and ADTCH3 presents a value betweenthese, as exemplified by the two sodium salts [23,24].

The electronic spectrum of ADTCH3 is depicted inFig. 5, showing an absorption band at 368 nm and othersmall bands around 300 and 275 nm. The most intenseband can be attributed to a p – p* transition, probablylocated on the oxocarbon ring. The squarate ion presentsa very characteristic absorption spectrum in the UV–visibleregion with an absorption maximum of 269 nm and ashoulder at 249 nm, separated by around 20 nm(�3000 cm�1); this spectrum is ascribed to the Jahn-Teller

Fig. 5. Electronic spectrum of ADTCH3. See details in Section 2.

effect, arising from strong vibronic coupling with the firstdegenerate electronic state [25,26]. Aniline shows an elec-tronic transition at around 290 nm [27]. In ADTCH3 theJahn-Teller effect is not observed as in the squarate ionspectrum, and the comparison between absorption maximaof ADTCH3 and the squarate ion shows that the electronicenergy difference is smaller in the substituted compound,indicating that there is a more effective electronic delocal-ization in the squarain, due to the planarity of the ringspresent in the structure. X-ray crystal structure shows thatthe oxocarbon and aniline rings are coplanar and that theangle between them is 8.96�, which can contribute to anextension of the electronic delocalization.

Raman and infrared spectra of ADTCH3 are shown inFig. 6. This compound presents a fluorescence emissionand due to this the Raman spectrum was obtained in thenear infrared using 1064 nm excitation. Tentative assign-ments were made by comparison with the vibrational datain the literature of potassium squarate [28], bis(dimethyla-mino)squaraine [10] and aniline [29]. The main vibrationalmodes of squaraine are listed in Table 4.

The vibrational spectra of ADTCH3 show several bandsdue to the molecular asymmetry of this squaraine. Somecharacteristic modes of the cyclobutene ring are verystrongly affected by the substitution of the oxygen atoms.The CO stretching mode of the oxocarbon ring is observedaround 1790 cm�1 as a weak band in both the infrared andRaman spectra. The analogous spectra of squarate salts inaqueous solution [30] and in the solid state [31] show thisvibrational mode in a similar spectral region, indicatingthat the amine substitution in the oxocarbon ring doesnot affect this stretching mode to a great extent. The diani-linium derivative compound, which presents the oxygenatoms in the 1,2-positions of cyclobutene ring , shows thismode at 1793 cm�1 [16] and in the mono-substituted pyr-imidium-betaine derivative of squaric acid it can be seenat 1792 cm�1 [32]. These results confirm the similarity of

4000

Inte

nsi

ty (

arb

itra

ry u

nit

s)

Wavenumber / cm-1

500100015002000250030003500

Fig. 6. Vibrational spectra (upper: infrared and lower: FT Raman) ofADTCH3. See details in Section 2.

Table 4Vibrational wavenumbers (in cm�1) of squaraine molecules and the potassium squarate salt

DMASQ infrared K2C4O4 ADTCH3 Tentative

Infrared Raman Infrared Raman assignment

257w c(CO)280w

319m 315w d(CO)651s 654w Ring bending726s 692w 688m Ring breathing (ox)

997w 998m Ring breathing1099m 1088m 1091m 1096w 1106w m (CC)ox

1145w 1100w 1127w 1190w 1189w m (CC)ox

1221m 1223s 1220m m(CN)1253m m(CN)

1265m x-sensitive modes1391s 1397s m(CC)ox

1407s 1407w dsym (CH)1443s 1447sh m(CN)

1458s dasym (CH)1477sh Phenyl ring stretch.1505m 1506w CNH bending

1540s 1528s 1524s m(CC)ox + m(CO)1560s 1559s m(CC)ox + m(CO), ring stretch.1587s 1595s m(CCox) + m(CoxN), ring stretch.

1604s m(CCox) + m(CO)1611m 1615s Ring stretch.

1650m 1640m msym(CO)1747w 1711w 1802w 1728w masym(CO)

1790w 1792w m(CO)2931w 2931w 2935w msym(CH)2978w 2978w 2973w masym(CH)

3013m 3014w m(CH)3059m 3063m m(CH)3120w 3124w m(NH)3197w m(NH)

192 C.E. Silva et al. / Journal of Molecular Structure 831 (2007) 187–194

the CO stretching mode in oxocarbon and pseudo-oxocar-bon compounds. A weak band observed at 1728 cm�1 inthe infrared and 1640 cm�1 in the Raman spectra can betentatively assigned to the asymmetric and symmetricstretching modes of the carbonyl groups of the dimerformed by hydrogen bonds between the carbonyl and theamine groups in the solid state (Fig. 3a).

Single crystal data show that the average of the CCbonds of oxocarbon ring in ADTCH3 (1.452 A) is verysimilar to that observed in the potassium squarate salt(1.457 A) [22]; both compounds possess a C2h symmetryin the solid state, and based on this, the modes related tothe oxocarbon ring in the squaraine are tentatively assignedby comparison with the observed vibrational spectra of thepotassium squarate salt [28,31]. In this compound, twobands are assigned to CC stretching; in the infrared spec-trum of the potassium salt such bands are observed at1100 and 1088 cm�1, whereas in ADTCH3 they are seenat 1190 and 1096 cm�1, respectively. Inspection of theRaman spectra of the potassium salt show these bandsoccurring at 1127 and 1091 cm�1, whereas in ADTCH3the same bands appear at 1189 and 1106 cm�1. The differ-ences between these two bands in both techniques aresmaller in the potassium salt (12 and 36 cm�1) than inADTCH3 (94 and 83 cm�1). These results are in agreement

with the determined crystal structure, where the biggest CCbond distance difference is smaller in the potassium salt(0.024 A) than in ADTCH3 (0.032 A), implying that amore effective perturbation is present in the CC bond inthe squaraine structure than in the squarate salt. The tenta-tive assignment of the CC stretching mode for the bands at1189 (Raman) and 1190 cm�1 (infrared) in the spectra ofADTCH3 is based on the investigation of Lunelli andcoworkers of a similar squaraine [10,12]. Moreover, Itoand West [30] have shown, using normal coordinate calcu-lations and polarization measurements that the non-totallysymmetric modes such as CC stretching are the mostintense observed in the Raman spectrum of squarate ionaqueous solutions. Resonance Raman profile investiga-tions of oxocarbon ions in aqueous solution have indicatedthat this effect is due to a Jahn-Teller distortion [25,26,28].In the Raman spectrum of ADTCH3, which is less sym-metric than the squarate ion, a similar effect is notobserved. The ring breathing in the potassium squarateion occurs at 726 cm�1 as an intense band in the Ramanspectrum, while in ADTCH3 a similar band is not observedin this region. An intense band observed at 688 cm�1 is ten-tatively assigned to the CC bond deformation of the oxo-carbon ring, in agreement with the assignment of Kolevand coworkers [32] in their study of betaine squaric acid.

C.E. Silva et al. / Journal of Molecular Structure 831 (2007) 187–194 193

Due to the substitution of oxygen atoms by the aminegroups in the structure of ADTCH3 the ring breathingmode can be coupled to the CO and CN bond stretchingmodes.

To a first approximation, ADTCH3 can be considered asbeing composed of squarate anions possessing substitutiongroups in the 2,4-positions; these groups are the aniliniumand dimethylammonium ions. Inspection of Table 4 andcomparison with literature data [10,12,29,33] show that themost important vibrational bands of each one of the substit-uents are not affected by the presence of the bonded cyclob-utene ring. For instance, breathing, stretching anddeformation ring modes of the aniline group are observedin a similar region of the vibrational spectra of aniline(998, 692 and 1615 cm�1) [29]. For the N–CH3 species, thespectral region of the C–H stretching modes is complicateddue to uncoupled oscillators and a Fermi resonance withthe CH3 bond deformation overtones [33]. In general, theCH bond located at the trans-position to the electronic lonepair of the nitrogen atom appears as a weakened bond, whilethe CH bond trans to the CN bond is strengthened; theseeffects can be observed by the presence of a band near tothe lower limit of the CH3 modes in this spectral region[33]. This experimental observation can be achieved mainlyin secondary and tertiary amines, however, it is not observedin compounds in which the electronic lone pair is involved inchemical bonding, such as in urea and amine salts [33].ADTCH3 shows medium intensity bands at 2973 and2978 cm�1 in Raman and infrared spectra, respectively,and these can be assigned to the asymmetric stretching modeof the CH3 groups, whereas the symmetric mode appears at2935 and 2931 cm�1, respectively. In the infrared spectrumof dimethylaniline [33], where the electron lone pair effectis present, a medium band has been observed at 2800 cm�1

and a broad band in the aliphatic CH stretching region.The spectroscopic behaviour in a similar region is very differ-ent for ADTCH3, where only a very weak band is observedat 2801 cm�1, suggesting that the electron lone pair isinvolved in an electronically delocalized system. All theseresults are in agreement with the X-ray single crystal mea-surements, which indicate that the canonical structure a (inFig. 4) appears as the most important contribution to the res-onance structure in ADTCH3.

The symmetric deformation mode of the methyl group,in general, presents variations in wavenumber due to themodification of the force constant of the HCX oscillatorwhich contributes to this mode. However, the out-of-phasemode is less affected and generally occurs between 1471 and1410 cm�1. In the literature, there is a connection betweenthe symmetric and asymmetric deformations of the methylgroup, as represented by the equation [33]:

mðX� CH3ðasymÞÞ ¼ 0:25mðX� CH3ðsymÞÞþ 1105ðstd: dev:9 cm�1Þ

Applying this relation for ADTCH3, and taking intoaccount the band observed at 1407 cm�1 which has been

assigned as the symmetric mode, the calculated value forasymmetric mode is 1457 cm�1. In the infrared spectrumof ADTCH3 a broad band at 1458 cm�1 is observed withtwo shoulders, one on the low wavenumber side at1447 cm�1 and another on the high wavenumber sideat 1477 cm�1. In this region, aniline shows one band at1470 cm�1, which is related to the phenyl ring stretchingand representing an a00 symmetry mode [29]. In the same re-gion a similar band at 1422 cm�1 can be observed in asquaric acid and pyridine molecular complex, assigned tothe Cox–N stretching mode, where ox refers to the oxocar-bon ring [32]. Comparing these results with the ADTCH3spectrum, the band at 1458 cm�1 can be assigned to theasymmetric deformation of the methyl group, the shoulderat 1447 cm�1 to Cox–N stretching and that at 1477 cm�1 tothe phenyl ring stretching.

As it can be seen from Table 2, crystallographic data showthat the N–CH3 distances (1.457 and 1.464 A) are very sim-ilar and also close in value to that of the /-N bond distance(1.411 A), where / refers to the phenyl ring. In the same way,the Cox–N(CH3)2 and Cox–N bond distances are very similar(1.315 and 1.339 A, respectively). Based on these facts thepresence of two bands assigned to CN stretching can be pro-posed, since all bonds with similar bond distances are expect-ed to be similar oscillators. The infrared band at 1223 cm�1

and the Raman feature at 1253 cm�1 are tentatively assignedto the symmetric and asymmetric stretching bands of theCH3–N–CH3 group, respectively, whereas the bandobserved at 1506 cm�1 in the infrared spectrum is assignedto the CNH deformation of the anilinium group, which isvery characteristic of secondary amines [33].

The Raman spectrum of ADTCH3 shows an intenseband at 1604 cm�1 assigned to the stretching of CC + CObonds, which is observed at 1611 cm�1 in potassium squa-rate [22]. The intense band at 1595 cm�1 is assigned to thestretching modes of CC + CN bonds, which is observed at1587 cm�1 in other squaraines investigated by Lunelli andcoworkers [10,12].

In the high wavenumber spectral region are observed thebands related to CH stretching of dimethylamine groups(2800–3000 cm�1) and to the phenyl ring at 3000–3100 cm�1 in both Raman and infrared spectra. Only oneband is expected for NH stretching; however, the infraredspectrum shows a medium-weak intensity band at3120 cm�1 and a broad band at 3197 cm�1. Both of thesebands can be tentatively assigned to the in-phase andout-of-phase stretching of the NH bond in the dimer struc-ture. These results are in agreement with the X-ray crystaldata which show the formation of a dimer in the solid state(Fig. 3).

In summary, this investigation has shown the relevanceand reliability of the use of vibrational spectroscopic tech-niques coupled with X-ray structural analysis in the morecomplete understanding of an important chemical com-pound such as squaraine. Also, for the first time the Ramanspectrum and a complete vibrational analysis of an unsym-metrical squaraine are presented, making use of excitation

194 C.E. Silva et al. / Journal of Molecular Structure 831 (2007) 187–194

with a near infrared laser (1064 nm) to suppress the fluores-cence in the squaraine.

4. Conclusions

Crystal data structural analysis shows that in the solidstate the aniline group in ADTCH3 is protonated forminga zwitterionic form. The nitrogen atom of the aniline ringand the oxygen atom of the four-membered ring of anothermolecule are involved in a medium strength hydrogen bondforming a dimer, where the intermolecular N� � �O distanceis 2.880(2) A. The pseudo-oxocarbon ring is planar and theangle formed between this ring and the aniline ring is 8.96�,which provides an extended electronic delocalization in thesquaraine. In the crystal packing, two types of moleculararrangements can be observed forming a wave design par-allel to the c crystallographic axis, and the angle betweenthe molecular planes is 67�. The cyclobutene rings areinvolved in an effective p-stacking interaction, where theinterplanar and centroid–centroid distances are 3.24 and3.58 A, respectively.

Vibrational investigation of ADTCH3 shows that theCO stretching and aniline modes are little affected by sub-stitution. Single crystal data show that the average of theCC bonds of the oxocarbon ring in ADTCH3 (1.452 A)is very similar to that observed in the potassium squaratesalt (1.457 A); however, in this potassium salt a little moreelectronic delocalization in the oxocarbon ring is observed.In these compounds two bands are assigned to CC stretch-ing in the vibrational spectra. The differences between thesebands are smaller in the potassium salt (12 and 36 cm�1)than in ADTCH3 (94 and 83 cm�1), which is in agreementwith the single crystal structure determination where thebiggest C-C bond difference is smaller in the potassium salt(0.024 A) than in ADTCH3 (0.032 A). The Raman spec-trum of ADTCH3 shows an intense band at 1604 cm�1

related to the m(CC) + m(CO) modes and another intenseband at 1595 cm�1, that can be assigned to m(CC) + m(CN)modes, similar to that observed in squarate salts and inother squaraine spectra. In the infrared spectrum a medi-um-weak band at 3120 cm�1 and a broad band at3197 cm�1 are observed which can be tentatively assignedto in-phase and out-of-phase stretching modes of the NHbond. These results are in agreement with the X-ray crystaldata, showing the formation of a dimer in the solid state.

Acknowledgements

The authors are grateful to CNPq, FAPEMIG and FA-PESP for financial support and research scholarships and

to the Laboratorio de Espectroscopia Molecular – USP –Sao Paulo for the Raman facilities.

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